Currently, most large size munitions use chemical compositions to initiate the ignition of propellant in munitions. Chemical energy igniters utilize convective heat transfer to ignite propellants and produce relatively heavy constituent gases which travel slower and cool rapidly compared to plasmas. Drawbacks associated with chemical ignitors include composition instability, and ignition speed.
In an effort to overcome the drawbacks associated with chemical ignitors, a variety of electrothermal-chemical (ETC) ignition systems have been developed. While the electrothermal ignition systems have the potential to provide more consistent and more uniform propellant ignition, these systems require large electrical energy storage devices to power their operation. As such, electrothermal ignition systems are typically used only for high caliber, high velocity gas systems.
Plasma is an electrically conducting gas composed of ions, electrons, and neutral particles sufficient to support an electric field. Examples of plasma include a lighting bolt, a spark plug discharge, and a spark from a shorted electrical circuit. The core temperate of plasma is extremely high, 10,000-20,000 degrees Kelvin. High temperature plasma is a very effective radiative heat transfer device since radiation heat transfers as a function of the temperature raised to the 4th power. U.S. Pat. Nos. 4,494,043, 4,835,341, 4,889,605 and 5,425,570 show examples of plasma discharge systems for applications other than ETC ignition systems.
In guns that use an ETC ignition system, plasma is created between electrodes of injector devices to ignite propellants in a very short duration time period (0.5-3 milliseconds). The effluent from these plasma devices consists of high temperature, low molecular weight gases traveling at very high velocities. These hot gases are extremely effective at permeating and igniting highly packed propellant charges that can be difficult or impossible to ignite with traditional chemical igniters. For a more detailed background on ETC ignition systems, reference is made to Chaboki et al., Recent Advances in Electrothermal-Chemical Gun Propulsion at United Defense, L.P.(19_).
To provide uniform ignition of the propellant, conventional plasma ETC ignition systems are oriented along a central axis of the munition. U.S. Pat. Nos. 5,072,647, 5,231,242, 5,287,791, 5,675,115 and 5,945,623 show various conventional ETC ignition systems having a single ignition tube aligned along the central axis of the combustion chamber of the munition. U.S. Pat. Nos. 5,431,105, 5,425,570, 5,503,058, and 5,515,765 show various ETC ignition systems having an outer ignition tube that is aligned with the central axis of the munition and surrounds the combustion chamber. U.S. Pat. Nos. 5,503,081, 5,767,439 and 5,886,290 describe an annular ETC ignition system that having embodiments that show a continuous central ignition tube, an annular outer ignition tube or a segmented central ignition tube, all of which are aligned parallel with the central axis of the munition.
Other more complicated arrangements for ETC ignition systems have also been developed. U.S. Pat. No. 5,233,903 shows the use of multiple staged plasma ETC ignition systems oriented at an oblique angle to the barrel of the gun. U.S. Pat. Nos. 5,171,932, 5,355,764, 5,444,208 and 6,119,599 describe different arrangements for using multiple plasma ignition systems oriented parallel to the central axis of the munition in either a collinear manner along the central axis or in a distributed in a circle around the central axis to energize a propellant. U.S. Pat. Nos. 5,688,416, 5,830,377 and 5,880,427 describe a tapered plasma injector that is aligned with the central axis of the munition and includes an adjustable magnetic field coil to enhance the ignition of the propellant.
One significant limitation on the use of all of these plasma ETC ignition systems is that there is a trade off between lengthening the plasma injector to increase the impedance, thereby allowing higher current to be used for quicker ignition, and providing a balanced plasma discharge, and lengthening a tail-like guide intrusion of the munition to improve the flight characteristics of the munition. This trade off is necessitated because both the plasma injector and the guide intrusion are located along the central axis within the combustion chamber.
Some plasma ETC ignition systems have been developed for munitions that do not include tail-like guide intrusions in the combustion chamber. Typically, these types of plasma ETC ignitions systems have a separate chamber for the ignition system and for an oxidizer material or propellant material. U.S. Pat. Nos. 4,711,154 and 4,895,062 describe plasma ETC ignition systems in which an oxidizer chamber is positioned between the plasma ignition system and propellant and a flat-ended munition.
U.S. Pat. Nos. 5,898,124 and 5,988,070 describe a plasma ETC ignition system in which the oxidizer chamber is positioned between the plasma ignition system and the propellant with a flat-ended munition located forward of the propellant chamber that is separate from the oxidized chamber. In U.S. Pat. No. 5,225,624, a stageable plasma injector is positioned in a chamber forming a plasma incubation region that exhaust upon ignition into a propellant chamber to propel a flat-ended munition. In each of these cases, the plasma igniter is oriented along the central axis of the munition and extends into the relevant chamber that is to be initially ignited.
Although significant advances have been made with respect to the development of plasma ETC ignition systems for guns, it would be desirable to provide an improved plasma injector ignitor that is more efficient for igniting propellants for munitions, and particularly one that is better suited for use with larger caliber munitions having tail-like guide intrusions that extend into the propellant chamber.